(1) Field of the Invention
The present invention relates to an apparatus and a method for plasma processing.
(2) Description of Related Art
Inductively Coupled Plasma (ICP) sources generate inductive plasma through the inductive magnetic field, which in turn produces circular (vortex) electric field accelerating electrons to ionize process gases and sustain plasma discharge. The inductive plasma is usually characterized with medium to high plasma (electron and/or ion) density and low level of Radio Frequency (RF) potential fluctuations. Plasma chemical processes carried out in inductive plasma are fast and produce low ion damages to semiconductor wafers and devices. Well-designed inductive discharges operate in substantially wider range of discharge conditions (e.g., gas pressure and power) in comparison with Capacitively Coupled Plasma (CCP) sources.
It is known in the art that high power (up to several kilowatts) inductors carry high RF currents on the order of magnitude of about several ten amperes. Such currents according to Ohm's low generate high RF voltages of up to a few kilovolts distributed along the inductors, which cause the inductors to display capacitive properties. Therefore, the inductors can also be considered as a capacitive electrode interacting with the plasma. Inevitably, parasitic RF capacitive currents from those high voltage inductors are generated or irradiated into the discharge plasma, which produce RF fluctuations of the plasma potential. The RF fluctuations are especially harmful for plasma processing because of electrical damages to production wafers and generation of parasitic RF capacitive plasmas or RF sheaths in the process chambers. The parasitic capacitive currents from the inductors are the main cause of plasma and process non-uniformity, several kinds of damages to the process chambers (e.g., arcing) and production wafers, and substantial increase in RF power losses.
U.S. Pat. No. 5,965,034 discloses self-balanced inductors particularly as helical resonators that can generate a push-pull voltage comprised of both phase and anti-phase RF voltages. According to the RF push-pull structure, both phase and anti-phase capacitive currents are arisen in the plasma, which essentially cancel one another, obviating the capacitive problem. Nonetheless, due to impedance matching requirements for resonance, the helical resonators must fit the electrical wavelength at an excitation frequency. It is difficult to build efficient self-resonant inductors in a limited space of compact low-aspect-ratio (more or less flat) high density plasma reactors. Therefore it is desired to develop an intrinsically (self) balanced inductive plasma sources that would not require excitation at resonance.